IndexedDB: Remove obsolete FIXME about window.onerror integration

The comment was a copy/paste of email proposing something that didn't
make it into the IDB spec. Further investigation shows that onerror
integration isn't at all straightforward and possibly not
Web-compatible.

BUG=302010
TBR=dgrogan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/124043002

git-svn-id: svn://svn.chromium.org/blink/trunk@164456 bbb929c8-8fbe-4397-9dbb-9b2b20218538